The single-beam push-pull method of making a light beam emitted from a semiconductor laser and focused onto an information recording layer of an optical disc follow an information track is widely known. In the push-pull method, the light beam reflected and diffracted by the information track (the returning light beam) is detected in a photodetector by a light-receiving section divided into two light-receiving surfaces. The push-pull signal can be obtained as the difference between the signals detected by these light-receiving surfaces. The light beam can be made to follow the recorded track by having an actuator shift the objective lens in the radial direction of the optical disc in such a way that the push-pull signal approaches zero.
In the conventional single-beam push-pull method, however, when the objective lens is driven by the actuator so that it shifts in the radial direction of the optical disc, the position of the objective lens may become offset from the position of the photodetector. The light spot that illuminates the light-receiving surfaces is then centered at a position removed from the dividing line separating the two light receiving regions, and this is known to add a direct-current offset (referred to below simply as an offset component) to the push-pull signal.
A technique for canceling this type of offset is disclosed in, for example, patent document 1 (Japanese Patent Application Publication No. H08 (1996)-63778). The optical pickup disclosed in patent document 1 has a polarization hologram that separates the returning light beam reflected by the optical disc into zero-order light and ±1-order light. The separated zero-order light, +1-order light, and −1-order light are detected by corresponding light-receiving surfaces, and the difference between the +1-order light and −1-order light detection signals is used as the push-pull signal. The light-receiving surfaces that detect the +1-order light and −1-order light have areas such that they are not affected by the amount of movement that occurs if the objective lens moves relative to the photodetector, so a push-pull signal with no offset component can be obtained.
One means of expanding the amount of information that can be recorded on a single optical disc is to use a multilayer optical disc in which a plurality of information recording layers are disposed one above another, increasing the amount of information that can be recorded by a factor substantially equal to the number of layers. Dual-layer discs having two information recording layers as in the commercial DVD (Digital Versatile Disc) and BD (Blu-ray Disc: registered trademark) standards are already in use.
In an optical disc device that is recording on or reproducing from an optical disc of this multilayer type, in addition to the light reflected from the information recording layer selected for the recording or reproduction of information, light reflected from other information recording layers is detected as so-called stray light. To record information on or reproduce information from the desired information recording layer accurately and at high speed, it is necessary to find means of excluding this stray light as far as possible, to reduce its effect on the recording or reproducing process. In the tracking error detection system, in particular, the differential push-pull method is generally used to cancel the offset component arising from objective lens shift. In the differential push-pull system, the light beam emitted from the laser light source is split by a diffraction grating into three light beams, including one main beam and two sub-beams, and three light spots are formed on the information recording layer on the information recording surface of the optical disc. Information is recorded on or reproduced from the information recording layer by the light spot of the main beam, formed in the center; the light spots of the sub-beams formed on the two sides are used to generate a tracking error signal. The beams are separated by the diffraction grating in such a way that the light intensity of the sub-beams is much lower than the light intensity of the main beam. A problem has been that the light intensity of the sub-beams reflected from the intended information recording layer and the light intensity of the stray light reflected from the other information recording layers, especially the light intensity of the stray light due to the main beam, may be about the same, causing major variations in the tracking error signal due to the stray light reflected from the other information recording layers, and impairing the quality of the tracking error signal.
A readily conceivable way of further expanding the recording capacity of multilayer optical discs is to increase the number of information recording layers. It then becomes necessary to reduce the spacing between adjacent information recording layers, which tends to increase the light intensity of the stray light from information recording layers other than the intended information recording layer. In a dual-layer disc, for example, there is only one other information recording layer that can give rise to stray light, but in an N-layer disc, stray light arises from (N−1) layers, so the light intensity of the stray light tends to increase still further.
Techniques for mitigating the effects of such stray light are described in, for example, patent document 2 (PCT publication No. WO 96/020473), patent document 3 (Japanese Patent Application Publication No. 2008-198336), and patent document 4 (Japanese Patent Application Publication No. 2005-203090). Patent document 2 discloses an optical head device in which the photodetector light-receiving surfaces that receive sub-beam light are disposed in positions at which the main beams reflected from other information recording layers are not incident. In this optical head device the stray light component is not detected by the photodetector, so the quality of the tracking error signal is not impaired.